The present invention relates to a piezoelectric resonant filter containing thin-film piezoelectric resonators, and a duplexer containing such piezoelectric resonant filters.
In mobile communication apparatuses such as cellular phones that have been spread remarkably in recent years, both reduction in size and increase in working frequency have advanced year by year. For this reason, both reduction in size and increase in workable frequency have been required of electronic components used in the mobile communication apparatuses.
Some mobile communication apparatus has a duplexer for performing switching between a transmission signal path and a reception signal path so that one antenna can be used for both transmission and reception. The duplexer has a transmission filter for passing a transmission signal but cutting off a reception signal, and a reception filter for passing the reception signal but cutting off the transmission signal.
A surface acoustic wave filter has been often used as each of the filters in the duplexer in recent years. The surface acoustic wave filter can support frequencies up to several GHz and is characterized in that the size of the filter can be reduced compared with a ceramic filter. In the present situation, however, many technical problems still remain in order to adapt the surface acoustic wave filter to the frequency that will be worked as a higher frequency in the mobile communication apparatus in the future.
Therefore, a thin-film piezoelectric resonator called thin-film bulk acoustic resonator has recently attracted public attention (see below mentioned Patent Documents 1 to 8 and Non-Patent Document 1). The thin-film piezoelectric resonator is a resonator using resonance in a direction of the thickness of a piezoelectric thin film. In the thin-film piezoelectric resonator, the resonant frequency can vary according to the change of the thickness of the piezoelectric thin film. It is conceived that the thin-film piezoelectric resonator can support frequencies up to several GHz. The concept “resonant frequency” used in this specification includes an antiresonant frequency except the case where the term “resonant frequency” is used particularly in comparison with the term “antiresonant frequency”.
The thin-film piezoelectric resonator has a piezoelectric thin film, a pair of electrodes disposed on opposite surfaces of the piezoelectric thin film, and a substrate for supporting the piezoelectric thin film and the pair of electrodes. The substrate may have a cave provided so as to be opened at a surface opposite to a surface on which the piezoelectric thin film and the pair of electrodes are disposed (see Patent Documents 1 and 2). Or a gap may be provided between the substrate and one of the electrodes (see Patent Document 3). Or the piezoelectric thin film and the pair of electrodes may be disposed on the substrate with interposition of an acoustic multi-layer film without provision of the cave and the gap (see Non-Patent Document 1).
For example, a ladder-type filter is a filter using resonators. The ladder-type filter includes series resonators and parallel resonators for forming a basic structure. As occasion demands, the ladder-type filter may be constituted by cascade connection of a plurality of portions each having a basic structure.
If no measure is taken, the resonant frequency of the thin-film piezoelectric resonator generally varies according to temperature change. This property is hereinafter referred to as temperature characteristic of resonant frequency. The temperature characteristic of resonant frequency is exhibited because the elastic constant of a typical piezoelectric material such as ZnO, CdS or AlN used in the piezoelectric thin film varies according to temperature change.
For example, in a thin-film piezoelectric resonator using ZnO as a piezoelectric thin film material, the temperature coefficient of resonant frequency is about −60 ppm/° C. Incidentally, the temperature coefficient of resonant frequency means the rate of change of resonant frequency in accordance with temperature change.
As a method for bringing the temperature coefficient of the resonant frequency in the thin-film piezoelectric resonator close to zero, there has been heretofore known a method in which a thin film (hereinafter referred to as temperature compensating film) made of a material having a temperature coefficient of elastic constant inverse in terms of plus/minus sign to the temperature coefficient of elastic constant of the piezoelectric thin film material is added to the thin-film piezoelectric resonator (see Patent Documents 1, 2 and 4). Incidentally, the temperature coefficient of elastic constant means the rate of change of elastic constant in accordance with temperature change. For example, SiO2 can be used as the material of the temperature compensating film.
When the temperature compensating film is added to the thin-film piezoelectric resonator, the temperature coefficient of the resonant frequency of the thin-film piezoelectric resonator varies according to the thickness of the temperature compensating film. Accordingly, when the thickness of the temperature compensating film added to the thin-film piezoelectric resonator is optimized, the temperature coefficient of the resonant frequency of the thin-film piezoelectric resonator can be brought close to zero.
Patent Document 5 has described a technique which is used in a thin-film piezoelectric resonator having such a structure that a lower electrode, a piezoelectric thin film and an upper electrode are formed successively on a substrate and by which a film made of an electrically insulating material such as SiO2 is provided between a lead-out portion of the upper electrode and the piezoelectric thin film. This technique aims at reduction of capacitance between the lead-out portion of the upper electrode and the substrate.
Patent Document 6 has described a technique which is used in a piezoelectric thin-film resonator having such a structure that a lower electrode, a piezoelectric thin film and an upper electrode are formed successively on a substrate and by which a dielectric layer made of a dielectric material such as SiO2 is provided on the substrate so that the effective thickness of the dielectric layer varies according to the place. This technique aims at reduction of capacitance between the lower electrode/upper electrode and the substrate.
Patent Document 7 has described a technique which is used in a lattice filter containing a plurality of thin-film resonators and by which a film is provided on part of the resonators so that mass load can be applied on the part of the resonators. The film is provided for changing the resonant frequency of each resonator by a predetermined value. In Patent Document 7, silicon oxide has been described as an example of the material of the film.
Patent Document 8 has described a technique which is used in a piezoelectric thin-film resonator including a thin film made of SiO2 and by which a frequency exhibiting at least one of series resonance and parallel resonance is measured and the thickness of the thin film is changed so that the difference between the measured frequency and a reference frequency is minimized.    [Patent Document 1]
Japanese Patent Laid-Open No. 137317/1983    [Patent Document 2]
Japanese Patent Laid-Open No. 153412/1983    [Patent Document 3]
Japanese Patent Laid-Open No. 189307/1985 (pages 2 and 3 and FIGS. 3 and 4)    [Patent Document 4]
Japanese Patent Laid-Open No. 68711/1985 (pages 2 and 3 and FIGS. 3 and 4)    [Patent Document 5]
Japanese Patent Laid-Open No. 141813/1984 (pages 2 and 3 and FIGS. 3 and 4)    [Patent Document 6]
Japanese Patent Laid-Open No. 171822/1985 (page 2 and FIGS. 3 and 4)    [Patent Document 7]
Japanese Patent Laid-Open No. 64683/1997 (pages 4 and 5 and FIGS. 4 and 5)    [Patent Document 8]
International Patent Publication No. 2001-502136 (page 15 and FIG. 6A)    [Non-Patent Document 1]
Kiyoshi Nakamura et al., “Thin Film Resonators and Filters”, International Symposium on Acoustic Wave Devices for Future Mobile Communication Systems, Collected Papers, pp.93–99, Mar. 5–7, 2001
A ladder-type filter has frequency characteristic exhibiting a low frequency side attenuation extremum and a high frequency side attenuation extremum disposed on opposite sides of a pass band. The resonant frequency of parallel resonators coincides with a frequency exhibiting the low frequency side attenuation extremum. The antiresonant frequency of series resonators coincides with a frequency exhibiting the high frequency side attenuation extremum. Accordingly, in the filter using thin-film piezoelectric resonators as the series and parallel resonators, there is a problem that the pass band of the filter varies according to temperature change when the resonant frequency of each thin-film piezoelectric resonator varies according to temperature change.
In a duplexer, when the pass band of the transmission filter or the pass band of the reception filter varies according to temperature change, the following problem occurs. Incidentally, the following description will be made on the assumption that the frequency band of the transmission signal is lower than the frequency band of the reception signal. In this case, particularly variation in the frequency of the transmission filter exhibiting the high frequency side attenuation extremum and variation in the frequency of the reception filter exhibiting the low frequency side attenuation extremum become issues. This is because variations in these frequencies cause lowering of performance of the duplexer for separating the transmission signal and the reception signal from each other.
It may be therefore conceived that a temperature compensating film having an optimal thickness is added to each of the thin-film piezoelectric resonators included in each filter in order to bring the temperature coefficient of the resonant frequency of each thin-film piezoelectric resonator close to zero.
An SiO2 thin film often used as the temperature compensating film is however amorphous and has no piezoelectric characteristic. For this reason, when the temperature compensating film of SiO2 is added to each thin-film piezoelectric resonator, the electromechanical coupling factor of the resonators as a whole decreases as the thickness of the temperature compensating film increases. As a result, the pass band width of each filter including the thin-film piezoelectric resonators is reduced.
Heretofore, in a filter including a plurality of thin-film piezoelectric resonators, for example, a temperature compensating film has been provided on the whole of the filter at the time of addition of the temperature compensating film to each thin-film piezoelectric resonator. In this case, reduction in the pass band width of the filter due to the provision of the temperature compensating film cannot be suppressed.
As described in Patent Document 4, the temperature compensating film may be provided only on a partial region of the substrate including a region where the thin-film piezoelectric resonators are disposed. Also in this case, the temperature compensating film having a uniform thickness is provided for all the thin-film piezoelectric resonators in the filter. Accordingly, also in this case, reduction in the pass bandwidth of the filter caused by the provision of the temperature compensating film cannot be suppressed.
In the technique described in Patent Document 5, a film made of an electrically insulating material such as SiO2 is provided on a region except a vibration portion of a thin-film piezoelectric vibrator. For this reason, this film does not function as a temperature compensating film.
In the technique described in Patent Document 6, the effective thickness of a dielectric layer is selected to vary according to the place. The thickness of the dielectric layer provided on a region where a thin-film piezoelectric resonator is disposed is however uniform. When SiO2 is used as the material of the dielectric layer, the dielectric layer provided on the region where the thin-film piezoelectric resonator is disposed can function as a temperature compensating film. When this technique is applied to a filter including a plurality of thin-film piezoelectric resonators, the dielectric layer having such a uniform thickness is provided for all the thin-film piezoelectric resonators. For this reason, in this case, reduction in the pass band width of the filter caused by the provision of the dielectric layer cannot be suppressed.
In the technique described in Patent Document 7, a film giving mass load is provided for changing the resonant frequency of a resonator by a predetermined value. For this reason, even in the case where silicon oxide is used as the material of the film, the thickness of the film cannot be optimized to bring the temperature coefficient of the resonant frequency of the resonator close to zero.
In the technique described in Patent Document 8, the thickness of a thin film is selected so that the difference between a measured frequency and a reference frequency is minimized. For this reason, even in the case where SiO2 is used as the material of the thin film, the thickness of the thin film cannot be optimized to bring the temperature coefficient of the resonant frequency of the resonator close to zero.